I'd start at the pump. ASSUMING it's all mechanical transmission(?) then all this pump does is supply steering. Both steering and hydraulic pumps are supplied from a common suction and filter. If one pump works, the problem isn't filter or supply.
Loosen the two bolts holding the pressure side flange to the steering pump a couple threads. Start it up for a few seconds and see what happens. If the pump is working it will make a mess in a few seconds. These pumps usually don't loose prime while operating but who knows? If it doesn't produce after a few seconds, you might want to remove it and check the drive. The shaft/gear that drives the pumps has been known to snap the tangs that mate with the pump coupling.
